Attention turntable specialists:  I have a Phillips 222 turntable
(belt-drive) that plays beautifully, but has developed an unusual
quirk:  it won't stop spinning.  I can hit 'stop' as many times as I
want, but it keeps spinning.  This can (sometimes) be cleared by
turning the power on and off a few times.  It also occasionally continues
to turn at the very end of records (i.e. doesn't auto-stop) -- and
these are not locked-groove records!!

What is broken here?  Guesses on how much it might cost to fix?